The present invention relates to antennas and more particularly to a dispatch antenna providing vertical polarization and wideband, rapid tuning in an aerodynamically efficient package.
The communications systems requirements of modern aircraft have placed increasing demands on antenna design. As of yet, the need for efficient, high powdered transmission in the Very High Frequency (VHF) and Ultra High Frequency (UHF) band with vertical polarization from small aircraft has not been satisfactorily achieved. An ideal antenna would have an aerodynamically efficient shape, less than one square foot forward cross-sectional area, and be adapted for existing aircraft antenna pods. Such an antenna should have a substantially omnidirectional radiation pattern while maintaining a Voltage Standing Wave Ratio (VSWR) of less than to one (2:1) over a five to one (5:1) tuning range. In environments employing frequency agile modulation, such as frequency division multiple access, the ability to tune to a new frequency with minimum settling time is also important.
One approach to this problem has been to use an array of electrically very small antenna elements. Each element is carefully tuned using an external automatic matching system or a resistive network to reduce the VSWR. Such arrays, besides being fairly complex electrically, exhibit attendant mechanical design complications such as weight, physical size, and packaging to withstand device enviromental conditions such as shock and vibration. The arrays can be used at a newly tuned frequency only after the settling time of the matching system, typically milliseconds, has passed.
It is also known that a parallel plate antenna configured as a thin disk of dielectric material plated on both sides can be used as an edge slot radiator to achieve the required vertical polarization. Rows of diametrically opposed tuning posts can be used to set the operating frequency. See, for example, D. H. Schaubert, H. S. Jones, Jr. and F. Reggia, "Conformal Dielectric-Filled Edge-Slot Antennas with Inductive-Post Tuning," IEEE Transactions on Antennas and Propagation, vol. AP-27, No. 5, pp. 713-716 September 1979. The edge slot has the desired vertical polarization. However, the basic parallel plate structure exhibits a typical 2:1 VSWR bandwidth of only approximately three to five percent. Additionally, the disk shape is not particularly adaptable to existing aerodynamic pods.
Others have demonstrated tunable microstrip patch antennas for microwave frequencies, embodied as a patch of metal separated from a ground plane by a dielectric medium. Diametrically opposed short circuiting switches such as diodes are disposed between the patch and the ground plane, and selectively switched to control the antenna. See, for example, U. S. Pat. No. 4,053,895 issued Oct. 11, 1977 to C. S. Malagisi and also see D. H. Schaubert, F. G. Farrar, A. Sindoris and S. T. Hayes, "Microstrip Antennas with Frequency Agility and Polarization Diversity," IEEE Transactions on Antennas and Propagation, vol. AP-29, No. 1, pp. 118-123, January 1981. These antennas exhibit a 2:1 VSWR bandwidth of five to ten percent, so that a large number of diode shorts would still be necessary to cover the desired tuning range. Direct scaling of the microstrip patch over ground antenna to VHF frequencies requires a comparatively large ground plane, again difficult to conform to existing aircraft antenna pods.